1. Field of the Invention
This invention relates to a process for cracking a hydrocarbon charge under conditions of high conversion utilizing a catalyst containing mordenite and faujasite.
2. Description of the Prior Art
Zeolite materials, both natural and synthetic, have been known in the past to have catalytic capability for various types of hydrocarbon conversion reactions. Thus, crystalline aluminosilicates of the faujasite type, e.g., zeolite X and zeolite Y, have been widely used in cracking catalysts for converting petroleum charge stocks, such as gas oil, to commercially attractive yields of gasoline. Representative prior art describing such processes include U.S. Pat. Nos. 3,140,249 and 3,140,253 to Plank and Rosinski.
It has also heretofore been known to employ mordenite, another crystalline aluminosilicate zeolite, either in the natural or synthetic form, as a hydrocarbon conversion catalyst. Thus, the use of acid mordenite as a catalyst for a wide variety of hydrocarbon conversion reactions including cracking is described in U.S. Pat. No. 3,597,493 to Frilette and Rubin.
Mixtures of various zeolites, including those of the faujasite type and mordenite, particularly when at least one of such zeolites is employed in the hydrogen form, have also been described as being useful in the conversion of petroleum hydrocarbons. Thus, U.S. Pat. No. 3,830,724 to Schutt describes a hydrocracking process utilizing a catalyst having Group VIII and/or Group VIB metals incorporated into a mixed zeolite support consisting of channel pore structure and three-dimensional pore structure zeolites of low alkali metal content. Representative of the channel pore structure zeolites is mordenite and representative of the three-dimensional pore structure zeolites is faujasite, either natural or synthetic, i.e., zeolite X or zeolite Y. At least one of the mixed zeolites and preferably both are in the hydrogen form, e.g., a typical mixed zeolite is one of hydrogen mordenite and hydrogen zeolite Y.
U.S. Pat. No. 3,925,195 to Scherzer describes a mixture of rare earth hydrogen Y type zeolite and hydrogen or transition metal exchanged mordenite as a hydrocarbon conversion catalyst. This catalyst is indicated to be useful in cracking petroleum charge stocks under conventional conditions of cracking.
U.S. Pat. No. 3,769,202 to Plank and Rosinski describes a catalytic cracking process in which a catalyst composition is employed comprising a mixture of discrete particles of a crystalline aluminosilicate having a pore size of less than 7 Angstrom units and a crystalline aluminosilicate having a pore size greater than 8 Angstrom units.
U.S. Pat. No. 3,764,520 to Kimberlin and Voorhies describes a hydrocarbon conversion process in which the catalyst used is a mixture of two different aluminosilicate zeolites having different ranges of pore size, i.e., 6-15 Angstroms and less than 6 Angstroms.
It has heretofore been recognized, as reported by J. R. Murphy in the Oil and Gas Journal for Nov. 23, 1970, that while zeolite catalysts have the ability to crack naphthenes, paraffins, or side chains containing these compounds rapidly and with excellent selectivity, these catalysts are ineffective in selectively cracking aromatic nuclei components. It was pointed out by the above author that the inability of zeolite catalysts to crack aromatic-nuclei selectively indicates that the limiting or optimum conversion is largely dictated by the aromatic content and particularly the polynuclear aromatic content. Since the polynuclear aromatic content generally decreases with decreasing boiling range, the selectivity in cracking light stocks is better than in cracking heavy stocks from the same crude source. But the ability of zeolite catalysts to crack low-boiling material is not all beneficial since it extends to nonaromatics in the gasoline boiling range. Thus, conditions of cracking must be chosen carefully to avoid recracking of gasoline.
The above publication indicates that overcracking of gasoline can be minimized by using mild conditions which only partially crack naphthenes and paraffins during initial contact with the catalyst. Recycle of the unconverted materials is thereafter necessary to obtain complete cracking of naphthenes and paraffins. Unfortunately, the cracking of polyaromatics in the recycle cannot be avoided in this type of operation. In addition to the poorer yields realized by substantial unselective cracking of polyaromatics, the rate of coke buildup on the catalyst is faster which adversely affects zeolite availability and the yields from cracking the naphthenes and paraffins.
It is concluded in the above publication that the objective in cracking gas oil with zeolite catalysts should be to maximize paraffin and napthene cracking while minimizing the polynuclear aromatic cracking and that considering the composition in the catalytic gas oils from high-conversion operations with zeolite catalysts, it is not probable that extensive cracking of the highly concentrated polyaromatic components is either possible or desirable.